Chromecast don't use Miracast 99% of the time. The content provider builds and publishes a client app for the chromecast that gets downloaded and will download

Chromecast don't use Miracast 99% of the time. The content provider builds and publishes a client app for the chromecast that gets downloaded and will download and play the content directly, it does not bounce through your phone first. The expectation being if you're casting local content/screens, then it's probably Miracast.